1. Field of the Invention
The present invention relates to a touch panel used for operation of various types of electronic devices.
2. Background Art
In recent years, various types of electronic devices such as portable phones or car navigation systems have become more functional and diversified. This involves an increase of devices in which their respective functions are selected with a light-transmissive touch panel attached on the front surface of a display element such as a liquid crystal panel. Therefore, touch panels with high visibility and reliable operability are demanded. A user presses a touch panel with his/her finger, a pen or the like for operation to select a function while viewing the content of the display element on the back surface of the touch panel through the touch panel. The conventional touch panel is described hereinafter with reference to the drawing. In the drawing, a dimension in a thickness direction is enlarged to understand the structure easily.
FIG. 3 is a sectional view of the conventional touch panel. In the touch panel, light-transmissive upper conductive layer 23 made of indium oxide tin or the like is formed at a lower surface of film-like and light-transmissive upper substrate 21 made of polycarbonate or the like. Lower conductive layer 24, which is similar to upper conductive layer 23, is formed on an upper surface of light-transmissive lower substrate 22. A plurality of dot spacers (not illustrated) is formed of insulating resin at certain intervals on an upper surface of lower conductive layer 24. A pair of upper electrodes (not illustrated) are formed at both ends of upper conductive layer 23, and a pair of lower electrodes (not illustrated) are formed at both ends of lower conductive layer 24 in a direction perpendicular to the upper electrodes.
Meanwhile, the outer periphery of upper substrate 21 and the outer periphery of lower substrate 22 are bonded to each other by means of a bonding layer (not illustrated) coat-formed on the upper and lower surfaces of frame-like spacer 25, thus upper conductive layer 23 faces lower conductive layer 24 at a certain interval. Polarizing plate 26 is produced by laminating triacetyl cellulose or the like on an upper surface and a lower surface of polyvinyl alcohol with iodine or dye oriented. Polarizing plate 26 is laminated on an upper surface of upper substrate 21, so that the touch panel is constructed.
The touch panel constructed in this way is arranged on the front surface of a liquid crystal display element or the like, to be attached to an electronic device, and a pair of upper electrodes and lower electrodes are connected to the electronic circuit (not illustrated) of the device.
In the structure mentioned above, the user presses an upper surface of polarizing plate 26 with his/her finger, a pen or the like for operation while viewing the content of the liquid crystal display element on the back surface of the touch panel. Consequently, upper substrate 21 bends along with polarizing plate 26, thereby causing the pressed position of upper conductive layer 23 to contact with lower conductive layer 24. Then, the electronic circuit applies voltage to the upper and lower electrodes sequentially, and detects the pressed position owing to the voltage ratio between the electrodes. As a result, various functions of the device are selected.
External light such as sunlight or lamplight from above transmits through polarizing plate 26 first. At this time, any one of lightwave among an X-directional lightwave and a Y-directional lightwave perpendicular to the X-directional lightwave is absorbed into polarizing plate 26. Therefore, the external light becomes only linear polarized light in the direction, where the lightwave has not been absorbed, and enters onto upper substrate 21 through polarizing plate 26.
Then, this light passes through upper substrate 21 and reflects upward at lower conductive layer 24. Intensity of the reflected light is reduced by substantially half because of polarizing plate 26 discussed above. As a result, reflection exited from the upper surface of polarizing plate 26 decreases, so that the liquid crystal display element or the like at the back surface can be seen easily. Such a touch panel is disclosed in Unexamined Japanese Patent Publication No. 2002-297319, for example.
As mentioned above, according to the conventional touch panel, because polarizing plate 26 is laminated on the upper surface of upper substrate 21, reflection of the external light is prevented, so that visibility is improved. However, upper substrate 21 made of polycarbonate or the like has a heat shrinkage ratio of approximately 0.01% after being left for 24 hours at 85° C. On the other hand, polarizing plate 26, which is produced by laminating triacetyl cellulose or the like onto polyvinyl alcohol, has a heat shrinkage ratio of approximately 0.5%. Such upper substrate 21 and polarizing plate 26 are laminated and bonded to each other. Therefore, when the touch panel is used in an ambient environment with high temperature and humidity, the difference in heat shrinkage ratio causes downward warpage in the intermediate part of upper substrate 21. This tends to result in unstable contact between upper conductive layer 23 and lower conductive layer 24.